The present invention relates to solid state power amplifier circuits for use in communication satellites and other RF microwave power amplifier applications.
Solid state power amplifiers may use either bipolar or field effect transistors. A solid state power amplifier exhibits gain compression if the desired output waveform requirement exceeds the available drain supply voltage. To obtain optimal operating efficiency, solid state power amplifiers are typically operated at a moderately compressed operating point, while still maintaining acceptable linearity. With fixed DC bias, a reduction in efficiency and an increase in power gain results when the input RF signal level is reduced below that required to obtain compressed operation. Since the output devices are biased with both RF and DC voltages, the operating point of the amplifier is determined by a combination of these voltages.
Similarly, for an increase in input signal level there is an associated increase in output power and a corresponding decrease in gain as the amplifier is driven further into compression. In multicarrier systems, these changes in operating point, with the corresponding changes in gain, linearity and efficiency may have profound effects on the system operation. In particular, this compression characteristic degrades the performance of systems, where the power gains of a matched group of asymmetrically driven solid state power amplifiers must remain matched. One such example is a multibeam phased array transmit antenna, where each element in the array is driven by a separate solid state power amplifier. Since output power to the elements is not uniformly distributed to form a particular beam, the power loading in the solid state power amplifiers associated with a beam driven at a high power level unbalances the gains of other solid state power amplifiers associated with other beams driven at lower power levels. This interaction results in decreased isolation between beams.
Messerschmitt-Boelkow-Blohm (MBB) developed an amplifier called the PAMELA (Power Amplifier Module for highly Efficient and highly Linear Applications) to overcome the above-described problems. The PAMELA is described in Product Note, 4/89-SSPA 2 published by MBB Space Systems Group, Communications Systems Dept., and entitled: "PAMELA, Highly Linear, Highly Efficient Solid State Power Amplifiers for the Mobile Satellite Service". In the PAMELA, the operating point of the amplifier is dynamically controlled by its load impedance. Load variation is controlled by three or more individual amplifiers operating through impedance inverters into a common fixed load. Drive-level-sensitive diode phase shifters are used to adjust the combining phases of the individual amplifiers. The resulting impedance match as seen at each amplifier output is a function of power level. The resulting operating bias point of the amplifier varies as a function of the output impedance match and therefore the drive level. Efficiency is maintained over a range of drive levels to the accuracy of the sequential contributions of components in the open loop system. However, it is a disadvantage that the gain and linearity vary with input signal level.
It is an objective of the present invention to provide a solid state power amplifier that operates at a substantially constant efficiency over a wide range of input and output power levels. Another objective of the invention is the provision of a solid state power amplifier that maintains constant gain and linearity in addition to constant efficiency over a range of input and output signal levels. Yet another objective of the present invention is to provide a solid state power amplifier that has a dynamically adjusted and selected operating point.